Equipment operating at high temperature in a wide variety of domains such as the manufacture of electronic components, optical fibres, reactor blades, etc. uses different forms of carbon-based thermal insulation. The reasons for this choice are:                the refractory nature of carbon, which is solid up to 3000° C.;        the low thermal conductivity of low density carbon base materials;        a relatively low production cost;        the possibility of obtaining very pure carbon and consequently limiting possible contamination of elements being worked at high temperature, which is an essential aspect for processes related to the electronic industry.        
The most frequently used forms of carbon-based thermal insulation are:                bricks based on carbon powder bonded together by a binder derived from carbonation of a liquid rich in carbonaceous material (e.g. pitch, phenolic resin, etc.). These bricks are very economic and are widely used in metallurgy (blast furnaces, furnaces with atmosphere saturated in CO);        so-called flexible carbon fibre felts made from carbon and/or graphite fibres formed into low-density felts, typically with a density of 0.1 g/cm3 (100 kg/m3);        so-called rigid carbon fibre felts, made from carbon and/or graphite fibres, bonded together by binders derived from carbonation of a liquid rich in carbonaceous material, or by deposition of a pyrocarbon in the gaseous phase onto a fibrous preform to be consolidated. The densities of these rigid felts are between 0.1 g/cm3 (100 kg/m3) and 0.3 g/cm3 (300 kg/m3);        carbon black, contained in a chamber to form a compacted powder wall between the hot area to be insulated and a cold environment. Densities of compacted carbon black typically vary between 0.05 g/cm3 (50 kg/m3) and 0.2 g/cm3 (200 kg/m3);        finally, the last family consists of materials described in particular in patent U.S. Pat. No. 3,404,061, and comprises particles of expanded graphite compressed in the absence of a carbonaceous binder to obtain solid structures with densities typically between 5 and 137 pounds per cubic foot, in other words between about 80 kg/m3 and 2300 kg/m3. There are several means of obtaining expanded graphite particles. They are described for example in U.S. Pat. No. 3,404,061 (milling, attack of spaces between hexagonal reticular planes by oxidising or halogenated agent water impregnation, heating to a temperature of more than 100° C.) or in U.S. Pat. No. 5,582,781 (milling, immersion in liquid nitrogen then thermal shock). In general, expanded natural graphite is used. When the compression applied to expanded graphite particles results in a density of more than about 0.4 g/cm3 (400 kg/m3), flexible graphite strips having a good mechanical strength are obtained.        
Each type of thermal insulation mentioned above has advantages and drawbacks which make its use more or less suitable for the special needs of each process.
The invention relates particularly to thermal insulation structures based on compressed expanded graphite particles. Presently, these structures are not very widely used, in comparison with structures based on carbon fibres. They are two types of reasons against a broad distribution of these products, despite their very competitive thermal properties:    1) structures based on compressed expanded graphite particles are very fragile if their density is less than about 0.2 g/cm3 (200 kg/m3), structures with densities lower than this value are extremely fragile and are practically impossible to use;    2) the most natural solution to overcome this problem of extreme fragility is to increase the densities obtained after compression, but the result is then the loss of insulation performance;    3) techniques for manufacturing this type of structure are not very productive.
They generally involve the following steps:                an extremely lightweight expanded graphite powder is produced, typically with a density of 0.003 g/cm3 (3 kg/m3) to 0.005 g/cm3 (5 kg/m3);        this powder is placed in a compression chamber with an appropriate geometry to achieve the required shapes;        the powder is compressed until a solid with the required density is obtained.        The ratio between the density of the initial powder and the density of the finished product makes it necessary to stack powder to a height at least 40 times more than the thickness of the required insulation product. Thus, if the objective is to obtain a 10 mm-thick thermal insulation product (typical value for carbon fibre felts), a mould with a minimum height of 400 mm has to be filled and slow compression has to be applied over a distance of at least 390 mm. Therefore, the method is not very productive and quality defects are easily produced, caused by difficulties in “degassing” the powder during the compression phase and by material heterogeneity.            4) due to their fragility, structures based on expanded graphite particles that have been compressed with a low compression ratio have the defect that they release graphite particles that are not well bonded to the material mass. This causes undesirable pollution in the chambers to be insulated and this pollution is a particularly severe handicap in industries such as those dedicated to the manufacture of semiconductors, in which cleanliness is of overriding importance.
In order to obtain thermal insulation materials which are particularly suitable for high temperature furnaces operating in a non-oxidising atmosphere, some patents disclose the use of multilayer sheets comprising at least one flexible layer made of a material based on compressed expanded graphite particles.
Thus, U.S. Pat. No. 4,279,952 describes a composite structure containing two of the forms of carbonaceous insulation mentioned above; one flexible carbon fibre felt layer (density between 60 and 100 kg/m3) is bonded to at least one flexible sheet made of a material based on compressed expanded graphite particles with a density of between 600 and 1600 kg/m3. The bond between the two layers is made by a carbon-based binder, for example a carbonisable polymeric resin. However, this composite structure has disappointing thermal insulation properties considering its relatively high cost, and the risk of pollution of the chamber is not fully eliminated since flexible felts are sources of large amounts of dust. In particular, fibrils originating from the visible edges of the felts become detached, and they are very easily carried in the air because they are so small.
Another structure is proposed by U.S. Pat. No. 4,888,242. In this patent the layers in the multilayer are not intimately bonded to each other over their entire surfaces since some layers (made of materials based on compressed expanded graphite particles) are corrugated before being connected to other layers that remain flat; contact surface areas between layers are small which improves the thermal insulation properties of the multilayer thus formed. However, this type of structure is difficult to make. If it is to contain a small number of layers, then large amplitude corrugations are necessary which are firstly difficult to produce, and secondly weaken the structure due to compression forces applied perpendicular to the flat surfaces. In practice, small amplitude corrugations are necessary, therefore there will be a large number of corrugated layers to be stacked in the structure, which implies a lot of gluing. The structure finally obtained has a fairly high average density and fairly disappointing thermal insulation properties, considering its high manufacturing cost.
Finally, U.S. Pat. No. 6,413,601 describes a thermal insulation furnace jacket obtained by using a flexible strip made of a material based on compressed expanded graphite particles, the said strip being wound spirally in several layers. The layers are bonded to each other by a bonding material, typically a phenolic resin. Resin degassing problems during carbonation are avoided by inserting a material, which decomposes under the effect of heat between two layers of the spiral which are coated with phenolic resin. Typically, a paper sheet is used, and as the paper decomposes it creates diffusion paths through which gases derived from carbonation of the resin will escape.
The applicant has attempted to make under satisfactory economic conditions a thermal insulation structure that does not have the disadvantages mentioned above and that can be used in the manufacture of thermal insulation elements, particularly insulation for furnaces operating at high temperatures and in non-oxidising atmosphere.